Add logo image to demo navbar

Add a 48x24 logo image to the demo's navbar.  It's just a hand drawn "S"
curve with a drop shadow added in GIMP.
Add a subdirectory page to the demo
Fix navbar logo image path for subdirectories

Make the logo image path in the navbar relative to the location where
the page being rendered is located, so it will be "../_static/logo.png",
for instance, if the page is in a subdirectory one level below the root.
Merge pull request #80 from cdbennett/topic/fix-logo-relative-path

Fix logo relative path